Photoelectric tube



Nov. 10, 1931. A. J. MCMASTER ET AL PHOTO ELEC'PR I C TUBE Filed June 14, 1930 fizz/6222575. Qrck zJ/Ya/fizsier C M7195 I farson Patented Nov. 10, 1931 UNITED STATES PATENT OFFICE ARCHIE J. MCMASTER AND CHARLES E. IPARSON, OF CHICAGO, ILLINOIS, ASSIGNORS TO G-M LABORATORIES, INC., OF CHICAGO, ILLINOIS, A CORPORATION OF ILLINOIS PHOTOELECTRIC TUBE Application filed June 14,

Our invention relates to photo-electric tubes and to a method for making such tubes.

A photo-electric tube in order to meet the requirements of present day commercial use, should be sensitive over the entire visible spectrum of light, and particularly in the region of yellow and red light, since the ncandescent lamp commonly used in assoc1a tion therewith is a strong emitter of light of this order. In many cases it is desirable to provide photo-electric tubes which have a maximum sensitivity for particular colors or wave lengths of light. We have discovered a method of pre-determining the mainmum sensitivity of photo-electric .tubes for specific colors.

An object of the invention is to provide a new and improved photo-electric tube and method of making the same. V

A further object is to provide a photo-electric tube and method of making the tube having high sensitivity for all colors of light in the visible spectrum.

A further objectis to provide a tube having high sensitivity in the red and yellow region of the light spectrum.

A further object is to provide a new and improved light sensitive surface, and a method of preparing such a surface.

A further object is to provide a new and improved method for scavenging a photoelectric tube.

Other objects and advantages-will appear as the description proceeds.

It has been found that there are a large number of factors affecting the final characteristics of a photo-electric tube, each of which must be taken into careful consideration to obtain uniformly 00d results.

In order to eliminate t e haphazard construction of photo-electric tubes, and the selecting of a few tubes which have the proper characteristics, we have made elaborate experiments to determine the exact conditions necessary for obtaining uniformly good results. It has been found that theelectron emittin surface of a photo-electric tube should ie extremely thin. However, a thin photo-electric surface has .a low conductivity and it is difiicult to make good electrical con- 1930. Serial No. 461,118.

tact therewith, and therefore, we provide a conducting base such as copper, silver, magnesium, gold, or other low resistance metals underneath the photo-electric surface.

A complete understanding of the invention may be had by reference to the following description taken in conjunction with the accompanying drawings, in which,

Fig. 1 is a side elevation of a tube embodying the invention, and made in accordance with the method of the invention,

Fig. 2 is a similar elevation in which the tube is rotated through an angle of 90 degrees from that shown in Fig. 1, and,

Fig. 3 is a plan view of the tube.

In practicing the invention a base 6 is provided having an envelope 7 mounted thereon, and within the envelope is a stem or press 8 having a air of supports 9 and 11 mounted thereon. orwardly of these supports is a further support 12 which also serves as the anode of the tube. Supports 9 and 11 carry a semi-cylindrical plate 13 which is processed to provide a light sensitive surface. In order to prevent microphonic noises due to relative movement between the anode 12 and cathode 13, the upper portion of cathode 13 is provided with a support 14 which is mechanically connected with the upper end of anode 12 by means of an insulating insert 15, preferably of glass, which holds the cathode and anode in fixed relative positions.

Electrical contact is made with the anode and cathode through prongs 16, one of which is connected to the anode, while another is connected to the cathode. While theplate 13 may be made of the materials enumerated above, it is preferably made by silver plating a copper plate. Extreme precautions must be taken in electro-p'lating with silver on the copper plate in order to avoid the production of materials as the result of chemicalreactions, which will deleteriously affect the light sensitive surface which is later applied thereto.

We have found that the following method of electro-plating avoids the presence of undesirable ingredients in the finished plate.

A copper plate is first immersed in sodium hydroxide, after which it is boiled in clear water. The plate is then immersed in a dilute solution of nitric acid which produces a crystalline surface of a somewhat rough appearance. This rough structure produced thereby is highly desirable later in the oxidization of the plate, since the rough surface presents a greater area and oxidizes more readily and thoroughly than a smooth surface. The plate is then Washed in the water and dipped in sodium cyanide.

When the plate is removed from the cyanide solution it is immediately electro-plated, before allowing it to dry, since if it is left to dry it tends to leave uneven spots on the plate. The electro-plated plate is then mounted on the stem in the envelope 7 together with the anode as described above.

An alternative method of electro-plating the conducting cathode plate is to mount a filament 22 having a coating of silver oxide thereon, on the stem 8. By passing an electric current through the filament the silver oxide is decomposed into silver and oxygen. Further heating of the filament vaporizes the silver which deposits on the cathode. A screen or shield 23 may also be interposed between the filament and the front wall of the envelope to prevent the deposit of silver thereon.

Simultaneously with the mounting of the elementsin the envelope, a capsule or pellet 17 is mounted in the upper portion of the envelope on a support 18 which is secured to the upper end of the anode by welding or the like. The pellet consists of a disc 19 having a bead 21 of an alkali or alkaline earth metal salt mixed with calcium or some other element having a'higher aflinity for theradical or element of the salt than the alkali or alkaline earth metal, from which the alkali or alkaline earth metal is evaporated and deposited upon the cathode. If for instance we wish to make a caesium tube the pellet may consist of a mixture of caesium chloride and calcium. This mixture upon heating the pellet forms calcium chloride and liberates caesium. The particular alkali or alkaline earth metal salt used depends upon the characteristics desired in the tube.

The method of processing the silver plated If we wish to obtain a tube which is highly copper plate in the tube will first be described in connection with the caesium salts.

After the elements and the pellet of caesium salt have been mounted in the envelope, the envelope is placed on an evacuating pump and exhausted. At the same time the envelope or bulb is placed in an oven to heat the bulb and expel the gases therefrom, and also to preliminarily expel the gases from the elements within the bulb. The cathode is then inductively heated by means of a high frequency current coil which is associated therewith in such a manner that the major portion of the lines of flux link with the portion of the plateadjacent thestem in order to avoid over heating of the disc 19 sup-- porting the pellet. After the bulb and elements have been thoroughly heated and exhausted to remove the gases from the tube, the envelope and elements are allowed to cool while the evacuation is maintained.

lVhen the elements and bulb have been sufficiently cooled the disc 19 is inductively heated to liberate metallic caesium from the pellet carried thereby. The tube is then heated in an oven, allowing the caesium to be deposited on the cathode, but preventing it from collecting on the inner surface of the bulb. The heating of the bulb to prevent the collection of caesium thereon eventually causes a heating of the cathode, and since the caesium has a low vaporizing point an inert gas such as argon of approximately 2 centimeters of mercury pressure is admitted into the tube, causing the caesium to remain on the plate. The 'bulb and elements are then cooled, and after cooling the gas may be withdrawn without danger of the caesium leaving the cathode. After the inert gas has been withdrawn approximately 1 to 2 millimeters of oxygen, 'or less, is admitted and allowed to remain in the v bulb until the caesium is oxidized. After the oxidation of the caesium the remaining oxygen is exhausted from the bulb. It may happen that all of the caesium is not oxidized. -However, caesium oxide and caesiumare mutually soluble, which causes the cmsium which may be underneath the caesium oxide to dissolve and form a thin layer of adsorbed metallic caesiumfilm on the surface of the cathode.

- Instead of admitting an inert gas into the bulb to retain the caesium on the cathode plate while the plate is heated, and then admitting 7 oxygen to oxidize the caesium on the cathode after the plate has cooled. a mixture of oxygen and an 1nert gas in which the oxygen exerts a partial pressure of 1 to 2 millimeters or less may be admitted immediately. In this way the step of exhausting the inert gas before admitting the oxygen is eliminated. It is advantageous to have the oxygen and inert gas thoroughly mixed before admitting them to the bulb, since diffusion of these gases takes place rather slowly in the bulb. The further procedure is then similar to that described above in that the caesium is oxidized and the remaining oxygen and inert gas are exhausted.

It has been found that by making the stem 8 of the bulb of lead glass some of the excess caesium reacts with the lead glass stem as shown by a brownish discoloration of the stem, and aids in cleaning up or scavenging. the tube. The envelope is, however, made of lime glass since discoloration of the envelope would decrease its transparency.

The bulb may now be sealed off and is ready for use. However, in most cases it is found desirable to admit an inert gas such as argon, helium, neon, krypton, xenon, or even a less inert gas such as nitrogen, before the bulb is sealed off. Such gases increase the sensitivity of the tube due to partial ionization when an electron current flows in the tube, as is well known in the art. If it is found upon testing the tube, that it does not have the expected sensitivity, the sensi-v tivity may be increased by passing a glow discharge through the inert gas. By making the cathode positive it may be bombarded by electrons to increase the concentration of the light sensitive metal on the surface of the cathode, and by making the cathode negative, the concentration of the metal on the surface of the cathode may be decreased. By this method the requisite concentration of the light sensitive metal for maximum sensitivity is obtained and substantially uni-. form and predetermined results are obtained, which eliminates the waste and shrinkage found in following methods heretofore used.

The process may be carried out in a similar manner in using potassium to obtain a cathode having maximum sensitivity for shorter wave lengths of light by employing pellets of a potassium salt, or when it is desired to use alkaline earth metals instead of caesium, to provide a cathode having maximum sensitivity for still shorter wave lengths of light, the pellet-s are formed of alkaline earth metal salts as pointed out above.

An alternative method may be used omitting the silver plating of the cathode. Instead of electroplating silver upon a copper plate, the alkali metals may be directly deposited upon the copper plate and oxidized as described above in forming the light sensitive surface.

It will be understood that the nature and embodiments of the invention herein described and disclosed are merely illustrative and that many changes and modifications may be made therein without departing from the spirit and scope of the invention.

What we claim is new and desire to protect by Letters Patent of the United States is 1-- l. A method of making a photoelectric tube which consists of mounting a plate cathode and an anode in an envelope, depositing an alkali metal on the cathode, heating the envelope to prevent the deposit of said metal thereon, admitting an inert gas to retain said metal on the cathode, cooling the envelope and exhausting said gas, admitting oxygen into the envelope, allowing the alkali metal to oxidize, and expelling the excess alkali metal from the envelope.

2. A method of providing alight sensitive surface on a cathode of a photoelectric tube which consists of depositing an alkali metal on the cathode, and subsequently admitting oxygen to the tube to oxidize said metal, and expelling the excess of said metal.

3. A method of providing a light sensitive surface on a cathode of a photoelectric tube which consists of expelling the gases from the tube and cathode by heat and evacuation, cooling the tube and cathode, depositing an alkali metal on the cathode, heating the cell externally to prevent the metal collecting on the inner wall thereof, admitting an inert gas into the cell to retain the metal on the cathode, cooling the tube and exhausting said gas, admitting oxygen to the tube and allowing said metal to oxidize, and expelling the excess of said metal from the tube.

4 4. A method of providing a light sensitive surface on a cathode of a photoelectric tube which consists of depositing an alkali metal on the surface of the cathode, heating the tube to localize the deposit on the cathode, providing an inert gas pressure to retain the deposit on the cathodewhile the tube is heated, cooling the tube and exhausting said gas, oxidizing said alkali metal on the cathode, and expelling the excess of said metal from the tube.

5. A method providing a light sensitive surface on a cathode of a photoelectric tube which consists of mounting the cathode in an envelope, depositing an alkali metal on the surface of the cathode, heating the envelope to localize the deposit on the cathode, providing an inert gas pressure to retain the deposit on the cathode while the envelope is heated, cooling the envelope and exhausting said gas, oxidizing said alkali metal, further depositing alkali metal on said cathode, and expelling the excess of said metal from the tu e.

6. A method of providing a light sensitive surface on a cathode of a photoelectric tube which consists of mounting a silver surface cathode in an envelope, depositing metallic caesium on the cathode, heating the envelope to prevent the collection of caesium thereon, admitting an inert gas to retain the caesium on the cathode, cooling the envelope and removing said gas therefrom, admitting oxygen and oxidizin the caesium, exhausting the oxygen, and heating and exhausting the en- 5 velope to remove the excess caesium.

7 A method of providing a light sensitive surface onthe cathode of a photoelectric tube which consists of mounting a metallic cathode in an envelope, exhausting the envelope,

10 heating a silver compound in the envelope to liberate metallic silver, causing said silver to deposit on the cathode, oxidizing said silver deposit and coating said deposit with a light sensitive film of alkali metal.

8. A method of providing a light sensitive surface on the cathode of a photoelectric tube which consists of mounting a metallic cathode in an envelope, mounting a filament having a silver compound applied thereto in the 0 envelope, exhausting said envelope, heating said filament to liberate metallic silver and deposit it on the cathode, oxidizing said silver deposit and coating said deposit with a light sensitive film of alkali metal.

In Witness whereof we hereunto subscribe our names this 6th day of June, 1930.

ARCHIE J. MCMASTER. CHARLES E. PARSON. 

